Please download the attached sequential ZEMAX file on the last page of this article:

            F FILE:  “Fold_Mirror_Start.ZMX”

This is a simple infinite conjugate system with a 90 degree fold mirror followed by a lens:

This file, at first glance, may appear to be setup correctly. In reality, a closer look at the fold mirror indicates otherwise:

Rays appear to be tracing behind the fold mirror which, given the geometry of our system, is physically impossible. So, why, then, are rays tracing as shown?

The answer is in how the surface following the fold mirror is setup in the Lens Data Editor. Take a look at the Thickness column in the Lens Data Editor:

Notice that the Thickness of surfaces 3 and 4 is zero. This means that the next surface, surface 5, is co-located with surface 3.

Click anywhere on the row corresponding to surface 5 in the Lens Data Editor and observe the 3D Layout:

Notice that surface 5 is co-located with the fold mirror. The two surfaces are co-located because there is no thickness change between the two surfaces. Surface 5 has a different orientation than surface 3 due to the Coordinate Break (surface 4) between the two surfaces. This Coordinate Break completes the coordinate system transformation required to model a fold mirror correctly. Thus, surface 5 has the proper orientation but it is not positioned at the correct location. This results in the virtual propagation of rays behind the mirror.

How can this be fixed?